Update httpcore interface

[lovelydinosaur]

To align with encode/httpx#1522

• request -> handle_request, arequest -> handle_async_request
• ext -> extensions
• reason -> reason_phrase
• http_version, reason_phrase -> bytes
• headers is no longer optional
• stream is no longer optional
• extensions are no longer optional
• httpcore.PlainByteStream() -> httpcore.ByteStream()

The httpcore interface then matches the HTTPX 0.18 Transport API.

Readme example after this change looks like this:

---

Quickstart

Here's an example of making an HTTP GET request using httpcore...

withhttpcore.SyncConnectionPool() ashttp:
status_code, headers, stream, extensions=http.handle_request(
method=b'GET',
url=(b'https', b'example.org', 443, b'/'),
headers=[(b'host', b'example.org'), (b'user-agent', b'httpcore')]
stream=httpcore.ByteStream(b''),
extensions={}
)
body=stream.read()
print(status_code, body)

Or, using async...

asyncwithhttpcore.AsyncConnectionPool() ashttp:
status_code, headers, stream, extensions=awaithttp.handle_async_request(
method=b'GET',
url=(b'https', b'example.org', 443, b'/'),
headers=[(b'host', b'example.org'), (b'user-agent', b'httpcore')],
stream=httpcore.ByteStream(b''),
extensions={}
)
body=awaitstream.aread()
print(status_code, body)

[lovelydinosaur]

Right, let's push on, push on.